Engine exhaust temperature and after treatment device temperature may rise to undesirable levels when the engine is operated at high loads for an extended period of time. If exhaust temperature and after treatment device temperature are permitted to increase without bound, after treatment device degradation and engine degradation may occur. One way to limit engine exhaust and after treatment device temperature is to richen an air-fuel ratio supplied to the engine. The additional fuel carries heat from the engine cylinders and exhaust system components to the tailpipe so that the cylinders and the exhaust system temperature may be reduced. However, operating an engine with a rich air-fuel mixture can reduce vehicle fuel economy and may increase vehicle emissions.
The inventors herein have recognized the above-mentioned issues and have developed a method for operating a hybrid powertrain, comprising: opening a disconnect clutch and limiting a temperature that an exhaust after treatment device reaches via an action in response to a desired torque being less than the threshold torque and an after treatment device temperature.
By opportunistically opening a disconnect clutch during conditions where desired torque is at a lower level, it may be possible to cool an engine and exhaust after treatment devices at a higher rate without having to operate the engine with a rich air-fuel mixture. For example, a DISG may provide a desired torque during conditions where desired torque is low, while at the same time, the engine may be operated at a higher engine speed and a reduced cylinder air charge per cylinder cycle to cool engine and exhaust system components. Further, the engine may be operated at a stoichiometric air-fuel ratio during at least a portion of such conditions so that exhaust after treatment device efficiency may be improved while the after treatment device is being cooled.
The present description may provide several advantages. Specifically, the approach may increase after treatment device cooling during selected conditions. Additionally, the approach may reduce engine emissions while after treatment device temperature is decreasing.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.